Explain on the basis of electronic configuration:

Question

The electron affinity of S is +200kJxmol^-1 and of Cl is +348kJxmol^-1

 

 

Answer 1

First, take a look at the electron configuration of S and Cl.

S:  1s2 2s2 2p6 3s2 3p4

Cl:  1s2 2s2 2p6 3s2 3p5

Ok, now what is electron affinity? it is the energy given off when an electron is ADDED to a neutral atom in the gaseous state.  And by the way, they are negative values, not positive, so S should be -200 kJ/mole and Cl should be -348 kJ/mole.

Regardless, the fact remains that the electron affinity for S is LESS than that for Cl.  Why is this so?

 

Sulfur (S) has 1 less proton in the nucleus (16 protons) than does chlorine (Cl) which has 17 protons.  Screening of the outermost shell (n=3) is the same because they both have the 1s 2s 2p and 3s configuration.  Also, the outer shell (n=3) is the same distance from the nucleus in both atoms.  So, the only real difference is the attraction of the outer electron, the one being added, by the nucleus.  It is greater in Cl than in S, thus the electron affinity of Cl is greater than that of S.